AD9961-EBZ datasheet(43/60 Pages) AD

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AD9961/AD9963

Rev. 0 | Page 43 of 60

The circuit shown in Figure 60 shows a typical output circuit

configuration that provides a non zero bias voltage at the

TXCML pin. Resistance values of 499 Ω for RL and 249 Ω for

RCML produces a 2 V p-p differential output voltage swing with a

1.0 V output common-mode voltage and a voltage of 0.5 V

supplied to the TXCML pin. The 2 mA full-scale current flows

through the 249 Ω RCML creating the 0.5 V TXCML voltage. The

decoupling capacitor, assures a low ac driving impedance for

the TXCML pin.

TXIP

TXIN

AD9961/AD9963

–

VOUT

TXCML

RCML

Figure 60. Circuit for Setting TXCML Level Using RCML

Transmit DAC Output Circuit Configurations

The following section illustrates some typical output configu-

rations for the AD9961/AD9963 transmit DACs. Unless

otherwise noted, it is assumed that IOUTFS is set to a nominal

2.0 mA. For applications requiring the optimum dynamic

performance, a differential output configuration is suggested.

A differential output configuration can consist of either an RF

transformer or a differential op amp configuration. The trans-

former configuration provides the optimum high frequency

performance and is recommended for any application that

allows ac coupling. The differential op amp configuration is

suitable for applications requiring dc coupling, signal gain,

and/or a low output impedance.

A single-ended output is suitable for applications where low

cost and low power consumption are primary concerns.

Differential Coupling Using a Transformer

An RF transformer can be used to perform a differential-to-

single-ended signal conversion, as shown in Figure 61. The

distortion performance of a transformer typically exceeds

that available from standard op amps, particularly at higher

frequencies. Transformer coupling provides excellent rejection

of common-mode distortion (that is, even-order harmonics)

over a wide frequency range. It also provides electrical isolation

and can deliver voltage gain without adding noise. Transformers

with different impedance ratios can also be used for impedance

matching purposes. The main disadvantages of transformer

coupling are low frequency roll-off, lack-of-power gain, and

high output impedance.

TXIP

TXIN

OPTIONAL RDIFF

AD9961/AD9963

RLOAD

Figure 61. Differential Output Using a Transformer

The center tap on the primary side of the transformer must be

connected to a voltage that keeps the voltages on TXIP and

TXIN within the output common-mode voltage range of the

device. Note that the dc component of the DAC output current

is equal to IOUTFS and flows out of both TXIP and TXIN. The

center tap of the transformer should provide a path for this dc

current. In most applications, AGND provides the most conve-

nient voltage for the transformer center tap. The complementary

voltages appearing at TXIP and TXIN (that is, VIOUTP and

VIOUTN) swing symmetrically around AGND and should be

maintained with the specified output compliance range of the

AD9961/AD9963.

A differential resistor, RDIFF, can be inserted in applications

where the output of the transformer is connected to the load,

RLOAD, via a passive reconstruction filter or cable. RDIFF, as

reflected by the transformer, is chosen to provide a source

termination that results in a low voltage standing wave ratio

(VSWR). Note that approximately half the signal power is

dissipated across RDIFF.

Differential Buffered Output Using an Op Amp

A dual op amp (see the circuit shown in Figure 62) can be used

in a differential version of the single-ended buffer shown in

Figure 63. The same R-C network is used to form a one-pole,

differential, low-pass filter to isolate the op amp inputs from

the high frequency images produced by the DAC outputs.

The feedback resistor, RFB, determines the differential peak-

to-peak signal swing by the formula

VOUT = 2 × RFB × IFS

The minimum single-ended voltages out of the amplifier are,

respectively,

VMIN = VMAX − RFB × IFS

The common-mode voltage of the differential output is

determined by the formula

VCM = VMAX − RFB × IFS

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